Originally Posted by johnfranklyn
Other members have pointed out Kia and other manufactures get their figures often on a rolling road thus facing no windage. I find its general to be conned by different manufactures and fully expect it by now. Perhaps someone will publish honest figures one day?
The figures are obtained from an official test and are the only figures manufacturers are allowed to quote. It's misleading but the manufacturers are only doing what they're told.
As far as diesel versus petrol costs in this country I find it hard to understand. It did favour the diesel owner at one time but now it seems producing diesel is more costly than producing petrol. I believe I read in France it is more the other way around?
Diesel tax in France is lower.
Part of the problem is that we buy fuel by volume
which ignores energy density. Energy density (which effectively amounts to miles travelled) varies by 10% through the seasons, with diesel containing about 10% more energy than petrol. In other words, a diesel car doing 50mpg is almost directly equivalent to a petrol car doing 45mpg, all else being equal. The seasonal variation means that if the ticket price stays the same through the year, different amounts of energy are being purchased for the money.
So, when considering it in terms of units of energy purchased, it is correct for diesel to cost more then petrol, per litre. By rights, summer prices for both should be higher than winter, not the other way round.
By contrast, energy density by weight
is pretty constant, with only a small difference between petrol and diesel. Technically, it would be more appropriate to buy fuel by the kilogram and to track fuel consumption in miles per kilogram.